The most basic kind of occlumency is concentrating on a single image or concept so intensely that a legilimens can't see anything else, but for that to work you have to know there's a threat. More advanced practice involves detailed introspection to notice when one's thoughts are being nudged by an outside influence, and the cultivation of mental habits that give thoughts more metaphorical inertia and make them harder to modify. There are exercises, starting with a simple "stare at an object and don't think about anything other than that object for sixty seconds" and moving on to more sophisticated introspection, concentration, and thought-redirection techniques, and some ways to tell if you're doing them right, but the only way to be sure is having someone try something on you, ideally when you aren't expecting it, and seeing if you detect and/or block it.

That doesn't sound too hard. Cam makes himself a nice malachite sphere to stare at and meditate on for a while.

It feels about like the book said it would. He can do all the exercises with similar ease if he puts several hours into it, though he will at some point in there get kicked out of the library.

He will relocate to his assigned room.

Then he can practice as much as he wants. Late in the evening he gets a note under his door inviting him to join Harry at their first occlumency lesson the next day.

He will show up for that if it deigns to specify a time and place! (He is now wearing a mechanical watch.)

It does; the time is after breakfast the next morning and the place is Dumbledore's office again (also the password is now apparently "blood lollipop").

Harry is also there, looking a bit nervous.

"Hello, hello. Let us get started. I trust you both have had a look at the mental exercises?"

"I looked at a couple." He also had homework and needed a human amount of sleep.

Cam nods rather than specify in an unavoidably comparative fashion how much more he has accomplished than this twelve year old.

"Then the next step is to learn what a legilimency probe feels like. The best legilimens can be very subtle, but less sophisticated probes can be noticed if you know what to look for. This is also," he nods at Cam, "the aspect I am most confident is possible without our kind of magic. Now, does either of you wish to go first? I will of course exit your mind as soon as I have entered it and do my best not to look at anything too closely."

"Uh, what is your credence in the hypothesis that I have actually learned to do this overnight?"

"Oh, it's perfectly alright if you haven't; many people find they have more success with the exercises once they have a sense of what they're supposed to be countering."

"Sure," says Cam, "but I'd in fact rather you not read my mind, so if I'm likelier to manage this after another week..."

"You are of course welcome to wait another week, though be aware that many people reach a point of diminishing returns with the exercises until they've had some practice with a legilimens."

"Isn't there a less invasive option? The book mentioned a truth potion, which in principle with a cooperative interlocutor wouldn't be an issue."

"Ah, yes, veritaserum. Many people find the sensation of being compelled to speak the truth more unpleasant than that produced by legilimency, but if you wish to try that instead some can be procured. How about you, Harry? Do you have a preference?"

Shrug.

"I don't usually go around lying anyway and if you ask me invasive personal questions I can probably fill my mouth with caramel and then also tell everyone you did that."

Dumbledore seems amused at the mental image. "Very well, then. I believe our potions master has some in stock; I'll ask him to send it up." He writes a note on a bit of parchment, starts a heatless green fire in the fireplace, says "Professor Snape's office" and tosses the parchment in. 

"And now we wait. Harry, would you like to try the legilimency method in the meantime? I expect you in particular will benefit from being able to recognize legilimency."

"It's not going to hurt or anything, is it?"

"Certainly not. It will merely feel like your mind being drawn to the memory of, hmm, how about your breakfast this morning?"

Harry nods. "Okay."

"Excellent. Take a moment to clear your mind."

"Alright. I will make the attempt in three . . . two . . . one . . . now."

Harry blinks and shakes his head. "That did feel odd. It was like being reminded of eating breakfast but not being reminded by anything."